1. Field of the Invention
This invention relates to broadband antennas and, more specifically, to broadband antennas of compact size which are capable of receiving or transmitting multi-polarized electromagnetic radiation.
2. Brief Description of the Prior Art
Antennas are often required to receive or transmit electromagnetic radiation over several octaves of bandwidth while maintaining uniform radiation pattern and impedance characteristics within the operating band. Antennas of this type have been well known in the art for many years and include log periodic and spiral radiating structures. Often however, the polarization of the received electromagnetic signal is unknown and a conventional log periodic or spiral antenna may not respond to the sense of polarization being transmitted. The problem of responding to transmitted signals over a broad band for any sense of polarization (i.e. vertical, horizontal, left hand circular or right hand circular) is difficult and has not been completely solved in the prior art.
The most pertinent prior art of which applicants herein are aware is a patent to DuHamel (U.S. Pat. No. 4,658,262). This patent discloses a log periodic zig zag antenna having four identical zig zag members positioned 90 degrees apart. An RF processor consisting of two 180 degree Marchand baluns and a 90 degree hybrid, remote from the antenna, is used to feed a transmission line extending from a cavity in the base region of the antenna housing, upward along the antenna axis and attaching to the antenna central feedpoint.
A common failure mode of cavity backed antennas which are fed at the central feedpoint with a transmission line positioned on the antenna axis is that of mechanical separation between the antenna and transmission line. The failure usually occurs when the antenna is subjected to environmental stress such as thermal cycling or vibration. This problem exists because the thin circular antenna substrate, which is permanently attached to the cavity at its perimeter, acts as a diaphragm and moves up and down at the center (feed point region) due to thermal cycling and vibration. When this movement occurs, the antenna pulls loose from the transmission line attached to the central feedpoint, resulting in complete electrical failure. As will be demonstrated hereinbelow, the present invention eliminates this problem because the antenna transmission line is attached at the perimeter of the antenna (diaphragm) where there is no movement between the antenna and the feeding transmission line and, thus, there is far less stress at the antenna/feed connection interface.